1. Field of the Invention
The present invention relates to sine wave synthesis. More particularly, the present invention relates to digital generation of a sine wave signal at selected frequencies.
2. Description of the Prior Art
One of the many known methods of generating a sine wave signal is shown in block schematic form in FIG. 1. A resistive voltage divider network R.sub.A -R.sub.N provides various selectable fixed DC voltage steps. These voltage steps correspond to instantaneous sine wave curve values and are coupled to an analog multiplex (mux) circuit, such as analog mux 11. A decoder 12 operates with analog mux 11 to select in a repeating ascending and descending order the various steps along the voltage divider circuit to assemble the desired sine wave in proper sequence. A counter 13 receives a clock signal and produces timing signals that operate decoder 12 and that accordingly determine the rate at which the sine wave is generated, and therefore the frequency of the generated sine wave. A staircase sine wave approximation output from analog mux 11 is coupled through a low pass filter 14, by which voltage transitions along the curve are smoothed, to provide the output sine wave signal.
Such known sine wave generator circuits require complex arrangements of electrical components, some of which are analog and some of which are digital. It is very difficult to integrate such diverse circuit elements onto a semiconductor substrate. For example, known sine wave generator circuits require a digital-to-analog conversion input terminal for each voltage level of the generated sine wave. If the number of input steps are reduced, then waveform approximation is simplified, but the generated sine wave output signal more resembles a coarse staircase waveform than a sine wave. Such coarse staircase waveforms are of little use without significant complex filters. Such filters are difficult to integrate onto a semiconductor substrate without using considerable and precious space on the substrate, without adding expensive and time-consuming fabrication steps, and without causing waveform degrading amplitude, harmonic, and phase distortion.